The present invention relates to purple pigments based on glass frits having a coating containing colloidal gold. In a further aspect, the present invention relates to a process for the production of purple pigments and to their use for pigmenting glazes, enamels, decorative colors, glass colors, plastics, printing inks, lacquers and the like.
Purple pigments based on ceramic materials and colloidal gold can be obtained by various methods and have long been used in the production of ceramic decorations and for the pigmenting of plastics, lacquers and decorative colors.
Conventional processes for the production of purple pigments using fine-particle glass as the ceramic binder and colloidal gold as the color-producing component involve several process steps, namely: (a) precipitation of gel-form Cassius's gold purple from aqueous gold salt solutions with reducing agents, such as tin(II) chloride for example, followed by ageing and separation, (b) mixing of the freshly prepared, still moist gold purple with finely ground glass and grinding of the mixture, (c) presintering of the mixture at around 600.degree. to 800.degree. C., the gold particles being at least partly coated with glass, (d) fine grinding of the sintered material and, if necessary, adjustment of the desired color by addition of silver compounds and/or other fluxes.
Serious disadvantages of this process include the fact that separation of the gel-form gold purple from the aqueous solution in step (a) is extremely complicated, the fact that the color of the final pigment is influenced by the ageing of the gold purple in a manner that is difficult to reduce (for example browning in the case of partial drying) and the fact that two grinding steps which reduce the volume/time yield and an energy-intensive sintering step are required. In addition, the non-uniformity of the pigments thus obtained is visible under a microscope.
There has been no shortage of attempts to improve the process briefly described above. By precipitation of the gold purple in the presence of substances which form oxide hydrates, such as Al and Sn salts, is it possible to obtain more readily filterable co-precipitates of colloidal gold and the oxide hydrates, in addition to which the color of the co-precipitates is more reproducible and resistant to ageing. However, the other steps involved in the production process correspond to steps (b), (c) and (d) mentioned above, cf. for example RO Patent 64442 B (Chemical Abstracts 98 (26): 22 07 40e), so that the overall cost of the process is again considerable. Apart from this, the pigment is not uniform and the presence of the oxide can adversely affect the brilliance of the pigment and/or the properties of glazes, enamels, glass and decorative colors containing such pigments.
According to DD-PS 143 423, precipitation of the gold purple can also be carried out in the presence of inert material. The purple-colored moist deposit is then homogeneously mixed with a glass flux and, for color control, with silver carbonate and the resulting mixture is wet-ground, sintered at 650.degree.-680.degree. C. and re-ground. The purple pigments obtained by this process are also attended by the above-mentioned quality disadvantages.
Purple-coated pearlescent pigments based on platelet-like particles coated with metal oxides, such as in particular TiO.sub.2 -coated mica, with an optically semitransparent layer of gold (cf. DE-PS 32 29 837, DE-OS 38 25 702) or a surface layer of tin oxide in which colloidal gold is incorporated (DE-OS 37 31 174) are commercially available. In these materials, the gold coating is always situated on a metal oxide layer which first has to be applied to the platelet-like substrate (besides mica, glass is also mentioned). In no case is a particulate glass directly surface-coated with colloidal gold. The presence of a metal oxide layer beneath a gold-containing coating is clearly regarded as necessary. However, application of a metal oxide layer is complicated because coating with gold has to be preceded by a calcination step to convert oxide hydrates into oxides.
In addition, the metal oxides can adversely affect the quality of the pigment and its performance properties.